Police say they are "not giving up" hope of finding missing New Hampshire teen Abigail Hernandez, despite exhausting the best lead they had in the case -- a letter from the teen to her mom.

Authorities said Friday that the letter Hernandez sent her mom, postmarked weeks after she disappeared, was a "critical lead for law enforcement."

"It was one of the most tangible leads we had in this investigation and we wanted to take every step we could to follow through on that letter to try to find Abby," said New Hampshire Assistant Attorney General Jane Young at a press conference.

"As we stand here today we have not been successful in that task, but that does not mean that we are not giving up this search," she said.

The letter from Abigail Hernandez, 15, was postmarked on Oct. 23 and received on Nov. 6, according to the FBI and New Hampshire Attorney General's Office.

"When we received this letter, I will tell you it was unprecedented. It gave us hope," FBI special agent Kieran Ramsey said.

Investigators said they decided to come forward with the existence of the letter because they fear the teen may be in "grave" danger and could perhaps be used for sexual exploitation if she is still alive.

While it was unclear whether Hernandez left willingly or was abducted, Ramsey said investigators' "worst fear" is "someone may now be coercing her."

The existence of the letter was kept private at first, Ramsey said, because they wanted to glean everything they could from it and follow every possible lead.

Ramsey declined to reveal where the letter was postmarked or any of the letter's contents. He would also not disclose why it took two weeks from the moment the letter was postmarked for it to reach Hernandez's mother.

Young said investigators were able to verify the letter was from the missing teen because "the tone was a tone Abi would have used."

Young said revealing the existence of the letter may result in a deluge of fakes, however, "our hope is we see another letter [from Abi]," she said.

Hernandez was last seen on the afternoon of Wednesday, Oct. 9 leaving Kennett High School in North Conway.

When her mother Zenya Hernandez came home from her nursing job that night, her daughter wasn't home.

The FBI's search for the teen -- who was last seen wearing black yoga pants and a gray sweatshirt -- has been conducted by air, over rough wooded terrain, and in the nearby Sacco River.

Hernandez's last cell phone call was tracked to a spot near Cranmore Mountain, a ski resort near the missing girl's Village Way home, on the day she disappeared.

Two months later, Young said authorities will continue to chase every lead and asked for the public's help.

"We're asking you. Look at your neighbor. Look at the grocery store. Look at your church," she said. "Is there somebody you haven't seen here before?"